The present invention is directed to an expansion dowel including a dowel member with a leading end and a trailing end. The dowel member is sleeve-shaped at least for a part of its axial length from the trailing end. The trailing end is arranged to support a load. The outside circumferential surface of the dowel member is generally cylindrically shaped extending from the trailing end and the cylindrical shape is interrupted adjacent the leading end by an axially extending wedge-shaped recess having a depth decreasing toward the leading end. An expansion wedge is seated in the recess and projects inwardly into the bore formed by the sleeve-shaped part of the dowel member.
In expansion dowels there are two different types of expansion. Pre-expansion fixes the dowel and is effected by an expansion process when the dowel is placed into a receiving material and before a load is supported on the dowel. Post-expansion takes place when a load is placed on the dowel after it has been expanded and, accordingly, is dependent on the loading applied to the dowel.
Aside from expansion dowels which pre-expand or post-expand, there are expansion dowels which effect both pre-expansion and post-expansion. In the post-expansion group there is a known expansion dowel including a sleeve-shaped dowel member with a wedge-shaped recess having a depth decreasing toward the leading end of the dowel and arranged to receive an expansion wedge projecting partly into the interior of the sleeve-shaped dowel member. Pre-expansion of such a dowel is effected by a threaded pin which can be screwed into a thread within the sleeve-shaped dowel member and bears against the rear end of the expansion wedge. If an axial tension force is applied to the trailing end of the dowel member, it is displaced axially relative to the expansion wedge and post-expansion takes place.
While this known expansion dowel is simple to produce and use, it has a significant advantage as a result of the wedge-shaped expansion member in that expansion occurs only in a plane extending through the long axis of the dowel. During expansion the pressure required to achieve an adequate anchoring value can exceed the compressive strength of the receiving material for the dowel. Such excessive pressure can cause spalling of the receiving material particularly if the dowel is inserted into the receiving material at a small distance from an edge. When dowels are inserted into the tension region of a structure, cracks may develop which could impede post-expansion of the expansion dowel and could lead to the dowel falling out of the receiving material.